1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a V-belt type continuously variable transmission, which is capable of controlling a line pressure and a secondary pressure in accordance with the oil amount balance of an oil pump.
2. Description of the Prior Art
Conventionally, a V-belt type continuously variable transmission (hereinafter referred to as “belt CVT”) using a V belt has been known as a continuously variable transmission which can be suitably applied to a vehicle. The belt CVT is constructed by winding the V-belt around a primary pulley and a secondary pulley, and groove widths of the primary pulley and the secondary pulley are variably controlled by oil pressure.
An oil pump is connected to an input shaft of the belt CVT, and a pressure regulating valve regulates an oil pressure generated by the oil pump to generate a line pressure. A first cylinder chamber and a second cylinder chamber are provided to the primary pulley and the secondary pulley, respectively. The line pressure is supplied to the first cylinder chamber via a shift control valve as a primary pressure, and a secondary pressure obtained by regulating the line pressure is supplied to the second cylinder chamber. The groove widths of the primary pulley and the secondary pulley are changed according to the oil pressures supplied to the respective cylinder chambers, and the gear ratio continuously varies according to the radius of contact between the V belt and each of the pulleys.
Further, the primary pressure receiving area of the first cylinder chamber and the secondary pressure receiving area of the second cylinder chamber are set to be equal (see for example, Japanese Laid-Open Patent Publication No. 2001-165293).
The conventional belt CVT as described above, however, has the problem that in the case when the shift range is switched from a D range to an N range and then switched to the D range while an accelerator pedal is kept OFF in a high vehicle speed range, the oil pump which generates basic pressure for the line pressure cannot generate a specified oil pressure due to a decrease in the engine speed in the N range. Therefore, the actual line pressure, primary pressure, and secondary pressure are equal to each other, and hence a high vehicle speed gear ratio cannot be maintained and a down-shift occurs.